@@ -0,0 +1,14 @@
+---
+'@objectstack/trigger-api': patch
+'@objectstack/lint': patch
+'@objectstack/skills': patch
+---
+
+feat(trigger-api): an `api` hook verifies the vendor signature its start node declares (`config.signature`)
+
+Clause-②: yes (widening)
+Metadata-protocol: none
+
+- **`@objectstack/trigger-api`.** `start()` reads the hook's one verifier with `apiTriggerVerifierOf(binding.config)` (`@objectstack/spec/automation`). A start node that declares its sender's scheme (`config.signature`: DocuSign Connect's base64 `X-DocuSign-Signature-1`, a Stripe-style `t=…,v1=…`, a GitHub-style `sha256=`) now admits a correctly signed post that was answered `401` before. That scheme REPLACES the `x-objectstack-signature` check for its hook: the declared header is read on both faces of the door (the self-hosted mount and `handleInboundHook`) with `Headers.get`, so a repeated field is its one joined value; the HMAC-SHA256 is computed under the key `config.secretRef` names (read at every post through the engine's reader, `503` while it has no value), over the raw body or `<t>.<raw body>`; each digest is decoded strictly to 32 bytes and compared with `timingSafeEqual`, any one of several `v1` elements matching (a key roll); and a timestamped scheme is held to `HTTP_SIGNATURE_TOLERANCE_SECONDS` from `@objectstack/core`. A body-only scheme has no replay window, as the contract states. A valid `x-objectstack-signature` admits nothing on such a hook, and a post without the declared header is refused. A declaration the spec answers `unusable` arms a hook that refuses every post before any secret is read and never falls back to `x-objectstack-signature`; a declared scheme bound without the engine's `config.secretRef` reader is refused at `start()`. Every failure is the same `401 INVALID_SIGNATURE` with its own cause in the server log (`declared-header-absent`, `declared-header-malformed`, `declared-prefix-missing`, `declared-no-digest`, `declared-mismatch`, `declared-outside-window`, `declared-unusable`), never the key, the header value or the body. A hook that declares no scheme is verified exactly as before.
+- **`@objectstack/lint`.** `validateFlowApiTriggerSecret` grades an `api` hook by that same reader: a usable declaration is ready on its `config.secretRef` alone, and an unusable one is the new error `flow-api-trigger-signature-unusable` (with its `os explain` entry), located at the key the spec refuses. It fires only on a declaration the flow parse already refuses, so no stack that passed before is refused now.
+- **`@objectstack/skills`.** The automation and api skills teach `config.signature` where they said a vendor-signed callback cannot be admitted.
@@ -575,6 +575,57 @@ would send.
 A flow definition, including an `http` node's `url` and `headers`, is served to every member who can read flows. A token, API key or signed webhook url written there is readable by all of them. Route an outbound credential by where it sits, and call the connector with a `connector_action` node instead — see [Connectors](/docs/automation/connectors#authentication). A credential in a header goes to a declarative connector with `bearer`, `basic` or `api-key` auth, whose `auth.credentialRef` names the secret. A key in the query string goes to `api-key` auth with `paramName`. No `credentialRef` variant carries a secret in the url path, so an incoming-webhook url (whose path is the secret) cannot be routed that way: call the service through a token-authenticated connector instead, such as the `slack` connector, whose bot token is supplied to the plugin by host code rather than written in the flow (it is registered by that plugin, not declared as a `connectors:` instance). Otherwise such a url is served with the definition. Only `signingSecret` and a start node's `secret` are kept out of a flow saved through the metadata API: the metadata save door stores each in a write-only, encrypted flow credential store, a read never returns it, and the engine reads it only to verify an inbound post or sign an outbound request. A flow that ships in a package names its inbound hook secret instead of carrying it: the start node's `config.secretRef` holds the name the deployment fills (an environment variable of that name in the open tier, resolved through the same credential resolver as a connector's `auth.credentialRef`), and the engine reads it at every post, so no credential is published with the package. A post that arrives while the name has no value is answered `503`, never verified. The reference is a name, not a credential: it is served as written, it may not sit beside a literal `secret`, and it may not name a variable in the platform's `OS_` namespace. A packaged flow that carries a literal keeps it in its package source; the literal is withheld when the definition is served, and a credential store row for that flow, where one exists, wins when the engine verifies or signs. `url` and `headers` are stored and served as written.
 </Callout>
 
+#### An inbound hook that verifies its sender's own signature — `config.signature`
+
+By default an `api` flow's hook admits a post only by ObjectStack's own
+`x-objectstack-signature`. A vendor that signs its callbacks its own way
+(DocuSign Connect's `X-DocuSign-Signature-1`, a Stripe-style `t=…,v1=…`
+header, a GitHub-style `sha256=` header) is admitted by declaring how it signs
+on the start node, as `config.signature`, beside the `config.secretRef` that
+names the key:
+
+```ts
+export const EnvelopeStatus = defineFlow({
+  name: 'envelope_status', label: 'Envelope status', type: 'api', runAs: 'system',
+  nodes: [
+    { id: 'start', type: 'start', label: 'DocuSign Connect', config: {
+      hookId: 'docusign',
+      secretRef: 'DOCUSIGN_CONNECT_KEY',
+      signature: { signs: 'body', header: 'X-DocuSign-Signature-1', algorithm: 'hmac-sha256', encoding: 'base64' },
+    } },
+    { id: 'end', type: 'end', label: 'End' },
+  ],
+  edges: [{ id: 'e1', source: 'start', target: 'end' }],
+});
+```
+
+`signs` says what the sender signs, and picks the form:
+
+| `signs` | The sender signs | The header carries | Replay window |
+|:--|:--|:--|:--|
+| `body` | the raw body alone (DocuSign Connect; GitHub-style with `prefix: 'sha256='`) | the one digest, after `prefix` when one is declared | none: the sender signs no time, so a captured post verifies again while the key is unchanged |
+| `timestamp.body` | `<t>.<raw body>` (Stripe-style) | `NAME=value` elements: one `timestampElement` (`t`) and one or more `digestElement`s (`v1`, one per key while the sender rolls its key) | the same 300 seconds as `x-objectstack-signature` |
+
+`header` is matched without regard to case; a field the sender repeats is
+read as its one value, the copies joined with `, `, so a body-only header sent
+twice does not verify. `algorithm` is `hmac-sha256`, `encoding` is `hex` or
+`base64`, and the digest must decode to the 32 bytes of the HMAC, compared in
+constant time over the body exactly as received. The key is only ever the one
+`config.secretRef` names, read at every post like any hook secret (`503` while
+it has no value); a literal `secret` beside `signature` is refused.
+
+A declared scheme **replaces** the `x-objectstack-signature` check for its hook:
+the hook has that one verifier, so a post that carries only a valid
+`x-objectstack-signature` is refused, and so is one without the declared
+header. Every refusal is the same `401 INVALID_SIGNATURE` an unsigned post
+gets, with its cause in the server log only. A declaration the flow parse
+refuses (an unknown algorithm, a reserved header such as `authorization` or
+`x-objectstack-signature`, a timestamped scheme without its elements) never
+falls back to the platform check: `os validate` reports it as
+`flow-api-trigger-signature-unusable`, and a hook armed with it admits no post.
+The declared keys are listed in
+[api-trigger-signature](/docs/references/automation/api-trigger-signature).
+
 **Script:**
 
 The built-in `script` executor never evaluates an arbitrary JavaScript string —
 
@@ -145,6 +145,7 @@ export {
   FLOW_TIME_RELATIVE_DESCRIPTOR_UNROUTABLE,
   FLOW_TRIGGER_UNROUTABLE,
   FLOW_API_TRIGGER_SECRET_MISSING,
+  FLOW_API_TRIGGER_SIGNATURE_UNUSABLE,
 } from './validate-flow-trigger-readiness.js';
 export type {
   FlowTriggerReadinessFinding,
 
@@ -2066,6 +2066,32 @@ const FLOW_API_TRIGGER_SECRET_MISSING_EXPLANATION: RuleExplanation = {
   ],
 };
 
+const FLOW_API_TRIGGER_SIGNATURE_UNUSABLE_EXPLANATION: RuleExplanation = {
+  rule: 'flow-api-trigger-signature-unusable',
+  covers: 'why a hook with a broken sender signature admits nothing',
+  paragraphs: [
+    'An inbound api hook may declare how a third-party sender signs its posts, in its start node\'s ' +
+      '`config.signature`: the header the sender writes its digest in, `hmac-sha256`, `hex` or `base64`, and ' +
+      'what is signed, the raw body (`signs: \'body\'`, as DocuSign Connect signs it) or `<t>.<raw body>` ' +
+      '(`signs: \'timestamp.body\'`, a Stripe-style `t=…,v1=…` header). The key is the one ' +
+      '`config.secretRef` names, never a literal. A declared scheme REPLACES the `x-objectstack-signature` ' +
+      'check for its hook: the hook has that one verifier and never reads the platform header.',
+    'The finding means the declaration is present and the spec refuses it: an unknown algorithm or ' +
+      'encoding, a reserved header name, a timestamped scheme without its elements, no `config.secretRef`, ' +
+      'or a literal `config.secret` beside it. The verdict names the key the spec refuses at; the flow ' +
+      'parse prints the refusal itself in full.',
+    'A broken declaration never falls back to `x-objectstack-signature` and is never admitted unsigned. The ' +
+      'automation engine\'s flow parse refuses the flow at registration, and a trigger handed it anyway ' +
+      'refuses every post to its hook with the one `401 INVALID_SIGNATURE` every unverified post gets, so ' +
+      'the flow never receives a post. Fix the declaration, or remove `config.signature` to verify ' +
+      '`x-objectstack-signature` under the hook\'s secret.',
+    'A usable declaration is ready: the hook verifies the sender\'s header under the named key, a ' +
+      'timestamped scheme inside the same 300-second window `x-objectstack-signature` has, a body-only ' +
+      'scheme with no replay window because the sender signs no time.',
+    FLOW_GATE_REACH,
+  ],
+};
+
 // `flow-trigger-unknown-object` lives in the same rule file but is not one of
 // the five: its verdict depends on packages a build cannot see, so it warns —
 // except under a package closure the runtime publish gate states, where the
@@ -5661,6 +5687,7 @@ export const RULE_EXPLANATIONS: Readonly<Record<string, RuleExplanation>> = Obje
   [FLOW_TRIGGER_UNROUTABLE_EXPLANATION.rule]: FLOW_TRIGGER_UNROUTABLE_EXPLANATION,
   [FLOW_TRIGGER_UNKNOWN_EVENT_EXPLANATION.rule]: FLOW_TRIGGER_UNKNOWN_EVENT_EXPLANATION,
   [FLOW_API_TRIGGER_SECRET_MISSING_EXPLANATION.rule]: FLOW_API_TRIGGER_SECRET_MISSING_EXPLANATION,
+  [FLOW_API_TRIGGER_SIGNATURE_UNUSABLE_EXPLANATION.rule]: FLOW_API_TRIGGER_SIGNATURE_UNUSABLE_EXPLANATION,
   [FLOW_TRIGGER_UNKNOWN_OBJECT_EXPLANATION.rule]: FLOW_TRIGGER_UNKNOWN_OBJECT_EXPLANATION,
   [FLOW_INPUT_UNKNOWN_OBJECT_EXPLANATION.rule]: FLOW_INPUT_UNKNOWN_OBJECT_EXPLANATION,
   [VALIDATION_RULE_REGEX_UNCOMPILABLE_EXPLANATION.rule]: VALIDATION_RULE_REGEX_UNCOMPILABLE_EXPLANATION,
 
@@ -12,6 +12,7 @@ import {
   FLOW_TIME_RELATIVE_DESCRIPTOR_UNROUTABLE,
   FLOW_TRIGGER_UNROUTABLE,
   FLOW_API_TRIGGER_SECRET_MISSING,
+  FLOW_API_TRIGGER_SIGNATURE_UNUSABLE,
   validateFlowApiTriggerSecret as validateFlowApiTriggerSecretUnrecorded,
   schemaRefusalHead,
 } from './validate-flow-trigger-readiness.js';
@@ -30,6 +31,7 @@ const NEVER_FIRE_IDS: readonly string[] = [
   FLOW_TIME_RELATIVE_DESCRIPTOR_UNROUTABLE,
   FLOW_TRIGGER_UNROUTABLE,
   FLOW_API_TRIGGER_SECRET_MISSING,
+  FLOW_API_TRIGGER_SIGNATURE_UNUSABLE,
   FLOW_TRIGGER_UNKNOWN_EVENT,
 ];
 const fired: Array<{ rule: string; message: string; hint: string }> = [];
@@ -1521,6 +1523,31 @@ describe('validateFlowTriggerReadiness', () => {
           ],
         },
       ],
+      // [#30009] A declared sender scheme the spec refuses: never-fire, like a missing secret.
+      [
+        FLOW_API_TRIGGER_SIGNATURE_UNUSABLE,
+        'error',
+        {
+          objects: [candidateObject],
+          flows: [
+            {
+              name: 'broken_vendor_hook',
+              type: 'api',
+              status: 'active',
+              nodes: [
+                {
+                  id: 'start',
+                  type: 'start',
+                  config: {
+                    secretRef: 'DOCUSIGN_CONNECT_KEY',
+                    signature: { signs: 'body', header: 'X-DocuSign-Signature-1', algorithm: 'hmac-sha1', encoding: 'base64' },
+                  },
+                },
+              ],
+            },
+          ],
+        },
+      ],
       // ── The controls. Both are hedged, and the hedge is the whole reason the
       //    promotion above is not "everything in this file is an error now".
       [
@@ -1753,6 +1780,89 @@ describe('validateFlowTriggerReadiness', () => {
   });
 });
 
+// [#30009] A start node that declares its sender's own scheme (`config.signature`)
+// is graded by the hook's ONE verifier, read through the spec's
+// `apiTriggerVerifierOf` (the reader the engine and the trigger share): ready on
+// the key `config.secretRef` names and nothing else, and never ready when the
+// reader answers `unusable`.
+describe('[#30009] api trigger readiness reads the hook\'s one verifier (config.signature)', () => {
+  const DOCUSIGN = { signs: 'body', header: 'X-DocuSign-Signature-1', algorithm: 'hmac-sha256', encoding: 'base64' };
+  const STRIPE = {
+    signs: 'timestamp.body', header: 'Stripe-Signature', algorithm: 'hmac-sha256', encoding: 'hex',
+    timestampElement: 't', digestElement: 'v1',
+  };
+  const vendorFlow = (config: Record<string, unknown>, type = 'api') => ({
+    name: 'vendor_hook',
+    label: 'Vendor hook',
+    type,
+    status: 'active',
+    nodes: [
+      { id: 'start', type: 'start', label: 'Start', config },
+      { id: 'end', type: 'end', label: 'End' },
+    ],
+    edges: [{ id: 'e1', source: 'start', target: 'end' }],
+  });
+  const judge = (flow: Record<string, unknown>) => cliFlowFamily({ objects: [candidateObject], flows: [flow] });
+  const rules = (flow: Record<string, unknown>) => judge(flow).map((f) => [f.rule, f.severity, f.path]);
+
+  it('a usable declaration keyed by secretRef is ready, on both arms, at the CLI and the runtime publish gate', () => {
+    for (const signature of [DOCUSIGN, STRIPE, { ...DOCUSIGN, encoding: 'hex', prefix: 'sha256=' }]) {
+      const flow = vendorFlow({ hookId: 'vendor', secretRef: 'VENDOR_WEBHOOK_KEY', signature });
+      expect(judge(flow), JSON.stringify(signature)).toEqual([]);
+      expect(runRuntimeAuthoringRules({ type: 'flow', item: flow }).errors, JSON.stringify(signature)).toEqual([]);
+    }
+  });
+
+  it('an unusable declaration is never ready: one error, at the key the spec refuses', () => {
+    const cases: Array<[string, Record<string, unknown>, string]> = [
+      ['an algorithm the spec refuses', { secretRef: 'K', signature: { ...DOCUSIGN, algorithm: 'hmac-sha1' } }, 'config.signature.algorithm'],
+      ['a reserved header', { secretRef: 'K', signature: { ...DOCUSIGN, header: 'x-objectstack-signature' } }, 'config.signature.header'],
+      ['a timestamped scheme with no timestamp element', { secretRef: 'K', signature: { ...STRIPE, timestampElement: undefined } }, 'config.signature.timestampElement'],
+      ['no key reference', { signature: DOCUSIGN }, 'config.signature'],
+      ['a literal secret beside it (an inline key)', { secret: 'whsec_inline', signature: DOCUSIGN }, 'config.signature'],
+      ['a literal secret AND a secretRef beside it', { secret: 'whsec_inline', secretRef: 'K', signature: DOCUSIGN }, 'config.signature'],
+      ['a secretRef that is not a usable name', { secretRef: 'OS_AUTH_SECRET', signature: DOCUSIGN }, 'config.secretRef'],
+    ];
+    for (const [label, config, keyPath] of cases) {
+      const findings = judge(vendorFlow(config)).filter((f) => f.rule === FLOW_API_TRIGGER_SIGNATURE_UNUSABLE);
+      expect(findings.map((f) => [f.severity, f.path]), label).toEqual([['error', `flows[0].nodes[0].${keyPath}`]]);
+      expect(findings[0]!.message, label).toContain(keyPath);
+      expect(findings[0]!.message, label).toContain(`type: 'api'`);
+      // Never the missing-secret id beside it: one verdict per hook.
+      expect(judge(vendorFlow(config)).map((f) => f.rule), label).not.toContain(FLOW_API_TRIGGER_SECRET_MISSING);
+      // And never let through at the runtime publish gate.
+      expect(
+        runRuntimeAuthoringRules({ type: 'flow', item: vendorFlow(config) }).errors.map((f) => f.rule),
+        label,
+      ).toContain(FLOW_API_TRIGGER_SIGNATURE_UNUSABLE);
+    }
+  });
+
+  it('a declared scheme is graded on its key reference alone: a withheld-and-stored literal secret does not make it ready', () => {
+    const flow = vendorFlow({ signature: DOCUSIGN });
+    const findings = validateFlowApiTriggerSecret(
+      { objects: [candidateObject], flows: [flow] },
+      { restoredCredentialPaths: new Set(['flows[0].nodes[0].config.secret']) },
+    );
+    expect(findings.map((f) => [f.rule, f.path])).toEqual([[FLOW_API_TRIGGER_SIGNATURE_UNUSABLE, 'flows[0].nodes[0].config.signature']]);
+  });
+
+  it('CONTROL: a flow that binds no api trigger is not graded by this rule, and a hook with no declaration keeps its own verdicts', () => {
+    expect(rules(vendorFlow({ secretRef: 'K', signature: { ...DOCUSIGN, algorithm: 'hmac-sha1' } }, 'autolaunched'))
+      .map(([rule]) => rule)).not.toContain(FLOW_API_TRIGGER_SIGNATURE_UNUSABLE);
+    expect(rules(vendorFlow({ hookId: 'vendor' }))).toEqual([[FLOW_API_TRIGGER_SECRET_MISSING, 'error', 'flows[0].nodes[0].config.secret']]);
+    expect(judge(vendorFlow({ secretRef: 'VENDOR_WEBHOOK_KEY' }))).toEqual([]);
+  });
+
+  it('`os explain flow-api-trigger-signature-unusable` says what a declared scheme is and why a broken one admits nothing', () => {
+    expect(FLOW_API_TRIGGER_SIGNATURE_UNUSABLE).toBe('flow-api-trigger-signature-unusable');
+    const text = explanationOf(FLOW_API_TRIGGER_SIGNATURE_UNUSABLE);
+    for (const fact of ['config.signature', 'config.secretRef', 'REPLACES', 'never falls back', '401 INVALID_SIGNATURE', '300-second']) {
+      expect(text, fact).toContain(fact);
+    }
+  });
+});
+
 describe('[#22161] one-line verdicts — the never-fire family', () => {
   it('every verdict the cases above fired is one line of at most 200 characters', () => {
     // The coverage control first: each of the five ids fired at least once, so
@@ -1798,9 +1908,10 @@ describe('[#22161] one-line verdicts — the never-fire family', () => {
     [FLOW_TRIGGER_UNROUTABLE]: ['`resolveTriggerBinding`', 'manual — nothing to bind', 'one higher than its bound count', 'RESOLVED type', 'record-{before,after}-{create,insert,update,delete,write}'],
     [FLOW_TRIGGER_UNKNOWN_EVENT]: ['record-{before,after}-{create,insert,update,delete,write}', 'bind-time warn', 'multi-event shape', '`record-after-write`'],
     [FLOW_API_TRIGGER_SECRET_MISSING]: ['ADR-0041', 'HMAC', '`registerFlow`', 'whatever the flow\'s `status`', 'never receives a post', 'withholds'],
+    [FLOW_API_TRIGGER_SIGNATURE_UNUSABLE]: ['REPLACES', 'never falls back', 'flow parse refuses the flow at registration', 'names the key the spec refuses at'],
   };
 
-  it('covers exactly the five ids', () => {
+  it('covers exactly the never-fire ids', () => {
     expect(Object.keys(MOVED).sort()).toEqual([...NEVER_FIRE_IDS].sort());
   });